In many fields of use and, in particular, in the electrical industry, it is necessary to coat a ferrous metal. Such a coating desirably performs the function of separating and purifying the ferrous material and reacting with surface silica in the steel to form an electrical insulating layer.
For example, in the transformer art, the cores of transformers are usually formed of a ferrous material, such as silicon steel, which may be provided with a preferred grain growth orientation through annealing to provide optimal electrical and magnetic properties. It is necessary to provide a coating on the ferrous material prior to the final high temperature grain growth anneal. This coating performs three functions, including: separating the various turns or layers of the coiled material to prevent their sticking or welding together during high temperature annealing; aiding in the chemical purification of the ferrous material to develop desired optimum characteristics of the metal; and forming on the surface of the ferrous material being treated a refractory-type coating which electrically insulates one layer of ferrous material from the next during its use as a transformer or an electrical apparatus such as a motor armature or the like.
In the present state of the electrical apparatus art, the most widely used coating for a ferrous-containing material is a coating of magnesium oxide and/or magnesium hydroxide. These coatings are generally applied to the ferrous material in the form of a slurry or suspension of magnesium oxide and/or magnesium hydroxide in water. These slurries or suspensions (slurry and suspension are used synonomously herein) comprise a quantity of magnesium oxide or magnesium hydroxide in water, and are mixed sufficiently for the desired application.
The inventor has found that improved magnesium oxide coatings are obtained using non-aqueous magnesium oxide and/or magnesium hydroxide slurries. The use of non-aqueous solvents to prepare magnesium oxide slurries for application to steel represents a novel approach which offers unexpected benefits, including reduction or elimination of "tight magnesia" and an improved glassy coating.
As set forth in U.S. Pat. No. 2,385,332, during heat treatment at suitable temperatures, magnesium oxide can be caused to react with silica particles on or near the surface of a previously oxidized silicon-iron sheet stock to form a glass-like coating. Such coatings are useful as interlaminary insulators when silicon-iron sheets are used in electrical apparatuses, as for example in the core of a transformer.
In the production of silicon steel for the magnetic cores of transformers, the steel is generally annealed to provide optimum grain growth orientation which develops the magnetic properties of silicon steel. This anneal, which is usually carried out in a dry hydrogen atmosphere at high temperatures, also aids in purifying the steel. During annealing, the magnesium oxide in the added slurry or suspension reacts with silica on the surface of the silicon steel to form a glass-like coating of magnesium silicate. This glass-like coating provides electrical insulation during the use of the silicon steel in electrical apparatuses.
U.S. Pat. No. 4,512,823 describes magnesium oxide compositions which eliminate "tight magnesia", or excess magnesium oxide which adheres tightly to the annealed coating (glass film) formed on silicon steels, while minimizing the hydration rate in the aqueous coating bath. More particularly, a portion of the magnesium oxide in the coating slurry or suspension reacts with the surface silica to form a glass-like magnesium silicate coating, while the unreacted portion remains as excess magnesium oxide which must be removed prior to further processing. Generally, this removal is accomplished by mechanical scrubbing with nylon bristle brushes or the like. After scrubbing, if there is a residue, it is termed "tight magnesia" and is undesirable. The method of the U.S. Pat. No. 4,512,823 utilizes admixtures of barium oxide, barium nitrate, chromium nitrate, or their hydrates with magnesium oxide in an aqueous slurry to minimize the formation of "tight magnesia", thereby improving the stacking factor of the steel and improving production yield by lessening the quantities of unacceptable steel caused by "tight magnesia" deposits.
The instant invention represents a distinct method for minimizing "tight magnesia". More particularly, non-aqueous slurries of magnesium oxide are added, instead of aqueous slurries, to steel prior to annealing. When the coated steel is annealed, "tight magnesia" formation is greatly reduced or eliminated, and the resulting glass-like film is improved.
The distinction between this invention and the prior art is that the instant magnesium oxide slurries are non-aqueous. The inventor has discovered that the formation of "tight magnesia" is related, by some mechanism, to the presence of water during annealing. The use of non-aqueous solvents eliminates the major source of water.